1. Field of the Invention
The present invention relates to zoom lens systems and in particular, although not exclusively, to a zoom lens system suitable for an image-capturing optical system in a digital still camera.
2. Description of the Related Art
With the recent advent of high-performance image pickup apparatuses (e.g., camera) using solid-state image pickup devices, (e.g., video cameras, digital still cameras, other image pickup devices as known by one of ordinary skill in the relevant art, and equivalents), optical systems in the image pickup apparatuses have been to use zoom lenses with higher aperture ratios and wide angles of view.
Since a camera of this type can have various optical members (e.g., a low-pass filter, a color compensating filter), that are disposed between the rear end of a lens system and an image pickup device, an optical system used in the camera is to have a relatively long back focal length (back focus). In the case of a camera with an image pickup device for color images, an optical system used in the camera requires good image-side telecentricity to avoid color shading.
Conventionally, various short-zoom two-unit lens systems have been proposed that include two lens units of a first lens unit having negative refractive power and a second lens unit having positive refractive power and that perform zooming by changing the distance between the first and second lens units. In such short-zoom optical systems, varying the magnification is realized by moving the second lens unit with positive refractive power, and compensating the displacement of the image position caused by the variations in magnification is realized by moving the first lens unit with negative refractive power. Such a lens system having the lens configuration made up of two lens units can have a zoom ratio of about 2×.
In order to make the entire lens system compact while achieving a 2× or more zoom ratio, three-unit zoom lens systems have been proposed that include three lens units in which a third lens unit having negative or positive refractive power is added to an image side of a two-unit zoom lens system and that realize a higher zoom ratio (see, for example, Japanese Patent Publication Nos. 7-003507 (corresponding to U.S. Pat. No. 4,810,072) and 6-040170 (corresponding to U.S. Pat. No. 4,647,160)).
Other conventional three-unit zoom lens systems have a long back focal length and a satisfactory telecentricity (see, for example, Japanese Patent Laid-Open Nos. 63-135913 (corresponding to U.S. Pat. No. 4,838,666) and 7-261083).
Still other conventional three-unit zoom lens systems include a relatively small number of lens elements and offer a wide angle of view (see, for example, Japanese Patent Laid-Open Nos. 2-118509, 6-201993, 8-327907 (corresponding to U.S. Pat. No. 5,715,097), 10-213745, 2001-208969 (corresponding to U.S. Pat. No. 6,396,642), and 2003-177316 (corresponding to U.S. Patent Application Publication No. 2003-138245)).
Still other conventional three-unit zoom lens systems include a first lens unit which includes two lens elements in which one is negative and the other is positive and a second lens unit including a combination lens effectively arranged (see, for example, Japanese Patent Laid-Open Nos. 2002-023053 (corresponding to U.S. Pat. No. 6,618,210) and 2002-090625 (corresponding to U.S. Pat. No. 6,618,210)).
Still other conventional three-unit zoom lens systems have a 3× or more zoom ratio (see, for example, U.S. Pat. No. 4,828,372 and Japanese Patent Laid-Open No. 4-217219).
Still other conventional three-unit zoom lens systems have a 3× or more zoom ratio and that include a relatively small number of lens elements (see, for example, Japanese Patent Laid-Open Nos. 10-213745, 2002-277740 (corresponding to U.S. Pat. No. 6,597,513), and 2003-21783).
Three-unit zoom lens systems designed for 35 mm film cameras are difficult to be directly used in image pickup apparatuses with solid-state image pickup devices because their back focal lengths are too long and their telecentricity is insufficient.
To realize both miniaturization of cameras and a higher zoom ratio of zoom lens systems, a collapsible zoom lens system is widely used which reduces the gap between the lens units when image-capturing operations are not performed so that the amount of projection of the lens system is decreased.
Generally, in the case in which many lens elements are used in each lens unit of a zoom lens system, the length of each lens unit along the optical axis is inevitably long. In addition, in the case in which the distance traveled by each lens unit during zooming and focusing operations is long, the overall length of the zoom lens system is unavoidably long and thus a desired collapsed length is not achievable, so that such a zoom lens system is difficult to be used as a collapsible zoom lens system.